1. Field of the Invention
The present invention relates to a reliable heretojunction bipolar transistor (HBT) and a method of producing the same.
2. Description of the Background Art
Prerequisites with an HBT are reducing capacitance between a base and a collector and lowering offset voltage for enhancing RF (Radio Frequency) performance. A decrease in capacitance between the base and the collector increases gain while a decrease in offset voltage reduces effective ON resistance to thereby increase power efficiency.
Another prerequisite is reducing the scatter of RF performance among HBTs. The accuracy of a junction area between the base and the collector is the key to low capacitance between the base and the collector, low offset voltage, and small scatter of RF performance. An accurate junction area is not achievable unless the amount of side etching of the base layer is controlled by promoting accurate processing of an emitter layer that over lies the base layer. While dry etching with high anisotropy is available for promoting accurate processing of the emitter layer, dry etching damages a device and deteriorates the characteristics and reliability of the device. It has therefore been customary to use isotropic dry etching or wet etching.
To reduce base recombination current, an HBT using GaAs has an emitter layer implemented as an InGaP layer. Photoresist or an oxide film has heretofore been used as a mask for processing an InGaP layer. Such a material used as a mask, however, cannot closely adhere to the InGaP emitter layer and causes the amount of side etching of the emitter layer to be scattered. To control the side etching of the InGaP emitter layer during processing of the base electrode, Japanese Patent Laid-Open Publication No. 2000-124226 uses an SiN film as a mask for processing InGaP. Even this kind of scheme fails to accurately lower offset voltage and capacitance between the base and the collector without any scatter.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-136159 and 10-178021.